Comments

Yes The clearest case of when you would need to be both a historian and a philosopher is when you write a history of philosophy. Expertise in both fields would also be highly valuable in writing philosophy of history. Apart from these two categories, the blending of philosophy and history (or the virtues of being both a philosopher and a historian) may vary.

Consider matters from the standpoint of history: When would a history (or a historian) be aided by philosophy?

Because one may write a history of any number of things (persons, events...) from a history of warfare to a history of agriculture, it may not be obvious when philosophy comes into play. Off hand, it seems that some philosophy will be inevitable in any history insofar as the history reflects a view (or a philosophy) of evidence, explanation, relevance, reasons and causes. But there are cases when philosophy seems more explicit as in a history of the French revolution versus a history of the first cities in the world.

From the standpoint of philosophy: When would philosophy (or philosophers) be aided by history? Some historical grounding seems important in even the most abstract philosophical matters (the philosophy of mathematics, for example), but there are philosophical questions that seem less sensitive to historical conditions. Arguably, questions about whether there can be an actual infinite does not depend (or depend significantly) on the time or place they are raised. Arguments about infinity are not merely of academic interest, but they have been employed by many philosophers (Jewish, Christian, Muslim) in arguments for the existence of God.